Parenteral solutions such as normal saline, 5% dextrose, etc., are commonly infused into a patient's vein to replenish liquid and correct electrolyte imbalances. This is often done with a 1-liter bottle suspended mouth downwardly above the patient and liquid flows by gravity from the bottle and through a flexible administration line connected to the bottle and then through a needle inserted into the patient's vein.
The bottle of parenteral solution being infused into the patient has been sterilized either at the manufacturer of the bottled solution or at the hospital where hospitals make up their own solutions. Likewise the tubular administration set connected to the bottle has also been sterilized. While the solution, bottle and administration set are sterile, the solution might pick up a minute amount of sterile particulate matter in the micron size range. These particles could come from various sources such as the bottle, closure, or inner surface of the administration set.
In the past it has been proposed to place a so-called "final filter" at a lower end of the administration set immediately before it enters the venous needle and the patient's vein. While it would be desirable to filter out all sterile particulate matter regardless of how infinitesimally small, as a practical matter filters that would not pass any particles at all also would not pass liquid. With a working practical filter it is desirable to filter out 90 percent or more of all the sterile particulate matter 5 micron or above in size from a sterile parenteral solution.
The problem with previous filters that filtered out 90 percent of all particulate matter 5 micron or above size was the physical size of the filter. These previous filters were roughly the size of a silver dollar and presented a large and cumbersome unit hanging on a needle stuck in the patient's vein. Most of the prior filters were of the "absolute" type, with a series of holes of a given size passing straight through the filter for conducting liquid through the filter while physically blocking particle passage. A woven screen is an example of an absolute filter. Thus to filter out particles of larger than 5 micron the holes must be 5 micron size or smaller. The problem with the absolute filter was that occasionally one hole would be larger than the others and allow some larger particles through. Therefore, an absolute filter rated for 0.45 micron pore size might only filter out 95 percent of all particles of 5 micron and larger. To get liquid to flow through a 0.45 micron related absolute filter a very large surface area was required. An area of 1 inch diameter was not uncommon. Another reason for the large size of prior filters was that they were of hydrophilic or readily "wettable" materials that had an absorbing effect on the liquid. While liquid would pass through such hydrophilic filters, large surface areas were required to get the liquid flow to a rate normally used in intravenous administration.